La Niña is an extreme phase of a climate cycle that occurs naturally. The climate cycle involved is a coupled ocean-atmospheric occurrence resulting from the interaction between the atmosphere and the surface of the ocean. Known as the Southern oscillation, this climate cycle includes El Niño on one extreme and La Niña on the other. La Niña is the cold phase of the cycle. A La Niña pattern exists when unusually cool sea-surface temperatures occur in the eastern and central tropical Pacific Ocean around the equator in the area between the International Date Line and the coast of South America.
Taken together, La Niña and El Niño generally are viewed by scientists as among the most powerful of weather phenomena on the planet, because they can affect the climate over more than half the Earth. On average, this cycle of cold surface ocean temperatures occurs every three to five years and, typically, lasts about nine to 12 months. Cold episodes are important because they disrupt the usual patterns of atmospheric circulation and tropical precipitation. The effect of the disruption of these patterns is to enhance the normal climate that prevails in affected regions of the earth.
During a La Niña, for example, an area such as the Pacific Northwest in the United States, where there usually is a wet winter, would have a winter that is wetter than normal. On the other hand, the more arid climates of the southwestern U.S. would be drier than normal, and the rest of the country would tend to experience unusually warm weather during a La Niña cycle. Southeast Asia and India probably would have abnormally heavy monsoonal rains, and eastern Australia could be wetter than usual. This weather effect extends as far north as western Canada, where it causes colder winters, and as far east as southeastern Africa, where the winter weather tends to become cooler and wetter.
La Niña also affects the intensity and position of the jet streams; this, in turn, affects both the track and intensity of storms. During this cold cycle of ocean temperatures, the chances of hurricane activity affecting the Caribbean and the U.S. increase, as does the likelihood that the storms will be more intense. In addition, a strong jet stream is a necessary ingredient for severe weather such as tornadoes. A change in the position of the jet streams affects which regions are most likely to experience tornadoes in the U.S.